fix(effectiveViewport): Make sure to raise the event when initializin…

…g multiple elements with EVP handlers

And in tests make sure to subscribe to EVPChanged in Loading intead of Loaded (was failing on UWP)

(cherry picked from commit 2ee3b50)

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Given_FrameworkElement_EffectiveViewport.cs

@@ -1120,12 +1120,12 @@ public EVPTreeListener(FrameworkElement root, FrameworkElement leaf)
 				_root = root;
 				_leaf = leaf;
 
-				leaf.Loaded += ElementLoaded;
+				leaf.Loading += ElementLoading;
 			}
 
-			private void ElementLoaded(object sender, RoutedEventArgs e)
+			private void ElementLoading(FrameworkElement sender, object args)
 			{
-				_leaf.Loaded -= ElementLoaded;
+				_leaf.Loading -= ElementLoading;
 
 				Subscribe(sender);
 
@@ -1155,7 +1155,7 @@ public EVPListener Of<T>()
 
 			public void Dispose()
 			{
-				_leaf.Loaded -= ElementLoaded;
+				_leaf.Loading -= ElementLoading;
 				foreach (var listener in _listeners.Values)
 				{
 					listener.Dispose();

src/Uno.UI/UI/Xaml/FrameworkElement.EffectiveViewport.cs

@@ -33,6 +33,7 @@ partial class FrameworkElement : IFrameworkElement_EffectiveViewport
 	{
 		private static readonly RoutedEventHandler ReconfigureViewportPropagationOnLoad = (snd, e) => ((_This)snd).ReconfigureViewportPropagation();
 		private event TypedEventHandler<_This, EffectiveViewportChangedEventArgs>? _effectiveViewportChanged;
+		private bool _hasNewHandler;
 		private int _childrenInterestedInViewportUpdates;
 		private IDisposable? _parentViewportUpdatesSubscription;
 		private ViewportInfo _parentViewport = ViewportInfo.Empty; // WARNING: Stored in parent's coordinates space, use GetParentViewport()
@@ -45,6 +46,7 @@ public event TypedEventHandler<_This, EffectiveViewportChangedEventArgs> Effecti
 		{
 			add
 			{
+				_hasNewHandler = true;
 				_effectiveViewportChanged += value;
 				ReconfigureViewportPropagation(isInternal: true);
 			}
@@ -321,8 +323,14 @@ private void PropagateEffectiveViewportChange(
 				+ $"| reason: {caller} "
 				+ $"| children: {_childrenInterestedInViewportUpdates}");
 
-			if (!isInternal && viewportUpdated) // We don't want to raise the event when we are only initializing the tree due to a new event handler
+			if (viewportUpdated
+				&& (
+					!isInternal // We don't want to raise the event when we are only initializing the tree due to a new event handler somewhere in sub tree
+					|| _hasNewHandler // but if we have a new local handler, we do need to raise the event!
+				))
 			{
+				_hasNewHandler = false;
+
 				// Note: The event only notify about the parentViewport (expressed in local coordinate space!),
 				//		 the "local effective viewport" is used only by our children.
 				_effectiveViewportChanged?.Invoke(this, new EffectiveViewportChangedEventArgs(parentViewport.Effective));